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1. The electrical responses of single smooth muscle cells of the guinea-pig taenia coli to external stimulation were studied in two times hypertonic solution and compared with the responses to intracellular stimulation.

2. Exposure to Krebs solution made two times hypertonic by adding sucrose abolished the mechanical movement and stopped the spontaneous electrical activity. The electrical response to stimulation was essentially similar to that in physiological solution.

3. When the tissue was placed between stimulating electrodes, the cells near the cathode were depolarized and produced spikes, while the cells near the anode were hyperpolarized and produced small spikes only with weak stimuli. The cells near the centre were not polarized but produced spikes with a frequency pattern similar to that near the cathode.

4. When both stimulating electrodes were put close together at one end of the tissue, the intracellularly recorded extrapolar polarization changed its polarity at 1-2 mm distance from the stimulating electrode. When an insulating partition was placed between the stimulating and recording site, the reversed polarity was no longer observed and the electrotonic potential spread decayed roughly exponentially with distance from the stimulating electrode. The time course of the electrotonic potential was similar to that predicted from the cable equation applied to nerve. The space constant was 1·68 ± 0·08 mm (S.E. of mean) and the time constant was 60-100 msec. The cable properties may be explained by assuming that many fibres, connected in series and in parallel, are aggregated as functional units.

5. The strength—duration curve was a simple hyperbola and the chronaxie was about 20 msec. The relation between extracellularly applied current and intracellularly recorded potential showed that membrane resistance decreased with depolarization and slightly increased with hyperpolarization. The spike was propagated in both directions at the same speed as in physiological solution (7·3 ± 0·7 cm/sec).

6. Long anodal current often produced electrical activity of low amplitude which seemed to be due to the spike activity near the cathode, because the same frequency modulation was seen in both activities, and external hyperpolarization reduced the size of the propagated spike. Cessation of a strong and long anodal current was followed by slow depolarization, about 1 sec in duration and up to 10 mV in amplitude, which sometimes triggered a spike.

7. The difference between responses to intracellular and to external stimulation may be explained by assuming that different parts of the cell membrane have different electrical properties. They may be: A, areas of close apposition between cells; B, areas capable of generating the slow component; C, an area capable of producing the spike, but less excitable.

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